2000A&A...362..730C

Recent R-matrix calculations of electron impact excitation rates in OIII are used to calculate electron temperature and density-dependent emission line ratios R₁=I(4363Å)/I(4960Å+5007Å), R₂=I(1661Å+1667Å)/I(4960Å+5007Å) and R₃=I(2322Å)/I(1661Å+1667Å), for a range of electron temperatures (7500≤T_e≤30000K) and densities (10⁴≤N_e≤10⁷cm^–3) applicable to gaseous nebulae. The ratio-ratio diagrams (R₁, R₂) and (R₁, R₃) should, in principle, allow the simultaneous determination of T_e and N_e from measurements of the OIII features in a spectrum. Plasma parameters derived for a sample of mid- to high-excitation planetary nebulae from (R₁, R₂) and (R₁, R₃) measurements, produced using a combination of ultraviolet spectra obtained with the International Ultraviolet Explorer (IUE) and optical data from a number of observing runs, are found to show excellent internal consistency. They also show, in general, good agreement with the values of T_e and N_e estimated from other line ratios in the nebulae, therefore providing observational support for the accuracy of the theoretical ratios and hence the atomic data adopted in their derivation.